Wireless receivers typically have three fundamental elements: antenna(e), a receiver front-end, and a demodulator, as depicted in FIG. 1. The receiver front-end filters (108) and amplifies (110) the received signal, before mixing 112 the signal down to an intermediate frequency (IF) using a reference signal source (e.g., oscillator) 114. The resultant IF signal is further filtered (116) and amplified (118) before being passed to the demodulator 106.
As shown, the demodulator 106 is generally comprised of a detector (120), perhaps coupled with a reference signal source (122) to recover the transmitted content, which may be filtered (124) before further processing or rendering. Those skilled in the art will appreciate that the type of demodulator required for a given receiver architecture is dependent upon the type of modulation used in producing the received signal.
One such modulation technique is frequency modulation (FM). Frequency modulation (FM) is a method of imposing content (e.g., analog or digital information) on a carrier signal by varying the instantaneous frequency of the carrier signal based on such content. Thus, to recover the content imposed on the FM signal, a receiver uses an FM detector. Simplistically, an FM detector identifies the instantaneous changes in the frequency of the received signal, and translates such changes to produce a signal representative of the original content.
A number of conventional FM detector architectures are known which will recover a representation of the imposed content, sometimes referred to as the baseband signal, from the received FM carrier. Two well-known FM detector architectures, the discriminator and the phase-locked loop (PLL) are generally depicted with reference to FIG. 2.
With reference to element 200, an example discriminator architecture is depicted generally comprising a mixer 204 coupled with an oscillator network generally comprised of a resistor, inductive, capacitive (RLC) network 206 tuned to a desired intermediate frequency (IF). The discriminator uses an RLC network with a coupling capacitor to achieve a phase shift that is dependent on frequency. The RLC is tuned such that the nominal phase shift at the IF is ninety degrees. Deviation in frequency from the center (IF) produces an output from the phase shifter that is a small deviation from the 90 degrees, thus producing an output from the mixer (multiplier) that is proportional to the phase change and, as such, proportional to frequency deviation from IF.
Those skilled in the art will appreciate that, while the discriminator may work well in discrete applications, the inclusion of an inductor makes it extremely difficult to implement in silicon (i.e., in an integrated circuit (IC) package). While the inductor can be synthesized from a capacitor using an op-amp circuit or transconductance (gm/C) based gyrators, they must provide a high Q inductor to obtain reasonable outputs from the mixer, and must be wide enough to respond to the spectrum of the signal. Often, these are competing interests in the case of a low intermediate frequency, since the IF is only a few times larger than that of the modulation bandwidth and, as such, the percentage occupied by the bandwidth is quite large. This implies a low-Q network, which then may not produce enough signal at its output to be practical.
Element 220 depicts an example phase-locked loop (PLL) architecture, comprising a mixer 224 coupled with a signal source 226 feeding off of a filtered 228 feedback loop. The PLL attempts to track the incoming signal frequency by controlling the frequency of the signal source (voltage controlled oscillator (VCO)) 226. Thus, the control voltage on the VCO 226 is the demodulated signal, presuming that the bandwidth of the loop filter 228 is wide enough to pass the modulation frequencies.
Those skilled in the art will appreciate that the PLL 220 is a complex system that requires: significant engineering to conduct stability analysis; that the VCO be accurately designed with tight tolerances; and a wide bandwidth in the filter 228 to obtain low distortion. The wide filter can cause stability problems as well as allow spurious signals to pass to the output, which serves as the control input of the VCO. The VCO produces a signal at the IF which could corrupt the high-gain IF stages when implemented in silicon.